1. Field of the Invention
The present invention relates to a curing agent comprising a polyisocyanate compound as the main component the polyisocyanate compound containing a uretidine dione dimer and an isocyanurate cyclic trimer which are prepared from a product obtained by partial urethane modification of hexamethylene diisocyanate, which will be hereinafter referred to as "HDI" and paint and adhesive compositions containing the curing agent.
2. Related Background Art
In the fields of paints, coating and adhesive, larger resource saving, lower public nuisance and higher safety have been considerably required, and paints and adhesives of high solid type, powdery or aqueous system or hot melt type have been regarded as promising paints and adhesives with no public nuisance. Particularly in USA, paints and adhesives of high solid type are more and more used as measures to reduce VOC (volatile organic compounds). In Japan, conversion to the high solid type is not so remarkable as in USA, but use of the high solid type has been taken into consideration owing to recent activities of local self-governing bodies toward environmental protection, level-up of paints now in use by conversion to the high solid type, and furthermore trends to a guarantee of quality of, for example, USA-made cars and Japan-made cars in the automobile industry.
Polyurethane paints and adhesives of one-package type and two-package type comprising an HDI-based polyisocyanate as a curing agent and acrylic polyol or polyester polyol as the main component have distinguished weathering resistance, chemical resistance, abrasion resistance, etc. and have been used in the fields of automobile and architecture outer coating and bonding. From the foregoing viewpoints even in such a polyurethane field a system of high solid type, in other words, low viscosity type, has been also required for reducing the necessary amounts of solvents for diluting the actual coating viscosity. Thus, an isocyanurate type has been recently more and more used owing to the good weathering resistance and the low viscosity as an HDI-based polyisocyanate curing agent rather than the urethane oligomer type based on polyfunctional alcohols.
However, the isocyanurate type is in a structure of three molecules of HDI with a tendency to form a higher molecular weight compound and thus has a limit to formation of a lower molecular weight compound. To obtain a lower molecular weight compound, the structure must be a uretidine dione structure based on two molecules of HDI, and such a lower molecular weight type inevitably leads to a decrease in the number of functional groups, resulting in poor curing property and drying property, that is, poor workability. When such a low molecular weight type is used in a two-component system, its compatibility with other resin is lowered and the service conditions are inevitably restricted.